Rennet is a mixture of the proteolytic enzymes pepsin and chymosin (rennin), which is usually obtained from the fourth stomach of young ruminants. While pepsin is also used in the pharmaceutical industry, both enzymes (pepsin and chymosin) are used for the coagulation of milk protein in the manufacture of cheese. Additionally, microbial rennet, which is naturally produced by certain microorganisms, has been used as a substitute for natural rennet in the cheese production for decades. Exposure to enzyme dusts has long been known to cause occupational immediate hypersensitivities. The present paper reviews the results of an evaluation of the literature data concerning occupational airway sensitisation due to natural and microbial rennet. Cases of specific airway sensitisation caused by rennet could be shown clearly by several studies. Positive skin prick and challenge tests as well as specific IgE antibodies have been described, thus suggesting an immunological mechanism.